Note 3 - Investment Securities - Amortized Cost and Estimated Market Value of Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Investment securities available for sale, amortized cost, 1 year or less	$ 528,253	$ 294,698
Investment securities available for sale, fair value, 1 year or less	529,421	295,255
Investment securities held to maturity, amortized cost, 1 year or less	57,015	70,378
Investment securities held to maturity, fair value, 1 year or less	57,392	70,602
Investment securities available for sale, amortized cost, over 1 to 5 years	1,062,469	1,104,775
Investment securities available for sale, fair value, over 1 to 5 years	1,116,754	1,122,391
Investment securities held to maturity, amortized cost, over 1 to 5 years	146,003	161,911
Investment securities held to maturity, fair value, over 1 to 5 years	149,824	165,126
Investment securities available for sale, amortized cost, over 5 to 10 years	1,142,837	670,595
Investment securities available for sale, fair value, over 5 to 10 years	1,190,058	683,277
Investment securities held to maturity, amortized cost, over 5 to 10 years	128,728	149,492
Investment securities held to maturity, fair value, over 5 to 10 years	133,290	153,724
Investment securities available for sale, amortized cost, over 10 years	26,769	33,489
Investment securities available for sale, fair value, over 10 years	28,423	34,465
Investment securities held to maturity, amortized cost, over 10 years	0	0
Investment securities held to maturity, fair value, over 10 years	0	0
Investment securities available for sale, amortized cost, subtotal	2,760,328	2,103,557
Investment securities available for sale, fair value, subtotal	2,864,656	2,135,388
Investment securities held to maturity, amortized cost, subtotal	331,746	381,781
Investment securities held to maturity, fair value, subtotal	340,506	389,452
Investment securities available for sale, amortized cost	814,969	938,303
Investment securities available for sale, fair value	843,714	943,458
Investment securities held to maturity, amortized cost	306,551	356,291
Investment securities held to maturity, fair value	315,884	354,844
Investment securities available for sale, amortized cost	3,575,297	3,041,860
Investment securities available for sale, fair value	3,708,370	3,078,846
Investment securities held to maturity, amortized cost	638,297	738,072
Investment securities held to maturity, fair value	$ 656,390	$ 744,296